Assessment of the Efficiency of Drones in Surveillance and Control of Desert Locust, Schistocerca Gregaria

Abstract

Desert locust is a migratory pest with ability to change from solitary to gregarious phase in response to suitable ecological conditions and has been listed as one of the most destructive pest in the world. Current methods of surveillance and control rely on the use of ground and aerial methods which presented some challenges despite being used successfully during the recent upsurge, hence the need to supplement them with suitable and affordable alternatives. Successful use of drones to manage other pests makes it an attractive and potential technology for desert locusts’ management. However, the available studies have neither optimized the key parameters nor addressed the standard operating procedures for desert locusts management. This study bridged the existing gaps by establishing the key parameters for surveillance and spraying of desert locusts using a drone. For surveillance, Dji Mavic 2 pro, was launched at five different heights (30 m to 110 m) to capture the images of locusts in the field and time taken, speed of the drone, spatial resolution and distance covered at each height was generated from the drone. AGISOFT software was used to stitch the images together to form Orthomosaic maps which were compared at different heights and generated parameters tested for correlation. To test the optimum height for spraying Metarhizium on the locusts using a drone; drone was flown at five different test heights; 2.5, 5, 7.5, 10 to approximate the droplet density and compare it to the standard droplet density recommended for desert locusts control. To assess the efficiacy of M. acridum and the effectiveness of drones in its application, 500 grams of Metarhizium (Novacrid) spore was mixed in 20 litres of diesel and 1 litres sprayed at different heights on the caged live locusts of different stages (3rd and 4th as well as the adults) arranged systematically in one hectare. They were monitored for twenty one days in a controlled room and their mortality determined. Except for the resolutions of pictures (cm/pixel) taken, other generated parameters were negatively correlated with flight heights. The results confirmed that orthomosaic images are clearer and well defined at a lower (30 m) compared to images obtained at flight height of 50 and 70m. Images obtained at higher flight height (90 and 110 m) are blurred with less defined details. This study confirms variation of droplet density between the tested heights (F4,40 = 7.2; p<0.001). Droplets density observed at 5m (75.3± 11.1), 7.5m (96.0± 29.4), 10m (40.2± 10.1) and 12.5m (24.8 ± 6.51) had a significant variation while the droplets density observed at 2.5m (152.2 ± 4.8) was significantly different from the droplets densities observed at all tested heights except for 7.5m. There was evidence of variation of standard droplet density and mean droplet densities recorded at different heights. Height of 10m agreed with VAR as recommended standard droplets density within 45 droplets/ cm2 range. Mortality varied among the locusts development stages within heights (F2,30 =25.71; p<0.0001) and between heights (F 4,30 =143.39; p<0.0001). Interaction between stage-height also had a significant effect on mortality (F 8, 30 =3.6745; p=0.004). Survival probability varied between heights for third instar (𝜒42= 56.84; p<0.0001), fourth (𝜒42= 54.17; p<0.0001) and adults (𝜒42= 47.57; p<0.0001). The study contend that relatively low flight heights (30–50m) and high flight heights (90 to 110m) above the ground are advisable for intensive and extensive surveillance respectively. Spraying desert locusts using a drone at any height below 10.0 m may lead to over-deposition of the pesticide, while heights above 10.0 m may lead to under-application. This study demonstrated that spraying a control agent from a specific height (10 m) is more effective than other heights tested. Despite all the developmental stages of the desert locust being susceptible to Novacrid®, the recommended target stage for management using this biopesticide is the 3rd instar stage because of the higher mortality rate and lower survival probability at this stage.